Add test for loading files not on filesystem

The new test demonstrates that currently when
a file exists at an older changeset, but not
on the current HEAD, the file is unable to
load.
Add assert to verify initial condition of missing file
Improve missing file test to verify contents of file
Only add file on stream_close() when repo is dirty

Fixes #17
Fix abuse of end() to pass a variable
Merge pull request #17 from donoftime/load-when-not-on-filesystem

Unable to load files not on filesystem at current HEAD